I'm guessing the reasoning is this: Most of the Melee who are having trouble with Reaper damage is getting taken down from damage that they can't avoid or mitigate in any reasonably timely fashion.
Horrid Wilting for example has at best a save to protect you for half damage. Having just recently gone into a Legendary Tempest Spine, and getting a Horrid Wilting from Sorjeck at R3 and taking 3-4.5k damage (so around 1.5k to 2k saved damage). That was still in excess of 500 HP above what my toon was able to absorb even had I made the save. Having around another 50% Health would have been "just enough" to take a hit from something like that assuming I don't scrimp on my Fort Save.
Even with high PRR and MRR to tackle the incoming damage (that can be reduced by it), you still rely on HP to abosrb the hit (they go hand in hand with each other). Therefore, the first thing to boost would be HP in this case. As it is only the first step in changes being done.
J1NG